Decoherence and the Quantum-to-Classical Transition , Maximilian Schlosshauer , Springer, New York, 2007. $99.00 (416 pp.). ISBN 978-3-540-35773-5
Almost a century after it first engaged physicists, the relationship between quantum mechanics and classical physics remains problematic. Early in the 20th century, Niels Bohr formulated the correspondence principle as a guide to help construct the laws of quantum theory. But until the 1980s there was scant experimental motivation for examining the physical mechanism of the transition between the microscopic quantum regime and the macroscopic classical domain.
When quantum mechanics was developed in the mid-1920s, questions regarding its connection with classical mechanics became urgent, because the measuring devices used to study atomic and subatomic quantum systems are governed by the laws of classical physics. Bohr promulgated the so-called Copenhagen interpretation, which became the central dogma for understanding quantum mechanics. He insisted on the logical primacy of the classical realm, where observable physical quantities such as position and momentum are operationally defined.
All along, though, there was a sense of dissatisfaction among quantum physicists. Rather than providing a unified description based on a seamless transition between the two regimes, physicists were compelled to treat the quantum world qualitatively differently from the classical domain. Remarkable advances in the past 25 years in atomic physics, quantum optics, and low-temperature physics have made it possible to exhibit quantum interference effects at mesoscopic and macroscopic scales, prompting the development of decoherence theory. Instead of describing the quantum measurement process as a discontinuous change of the state of the system—with its attendant reduction of the state, or collapse of the wave function—the decoherence model is based on the application of the Schrödinger equation to the system and the environment with which it interacts.
Decoherence and the Quantum-to-Classical Transition is based on Maximilian Schlosshauer’s 2005 doctoral thesis and expands on his 2004 article in Reviews of Modern Physics. Schlosshauer, now a postdoctoral fellow in Melbourne, Australia, has written an excellent monograph about what the best current thoughts are on the link between quantum and classical physics. The key is the decoherence-inducing, ever-present interaction between a quantum system and its environment, an interaction that can be surprisingly efficient and swift. The tools needed to elucidate the quantum-to-classical transition via the process of decoherence—that is, turning a pure, or coherent, quantum state into a statistical mixture—involve a full arsenal of the typical features peculiar to quantum mechanics: superposition, entanglement, nonclassical correlations, superselection, and so forth.
Although much of the book is quite technical, many sections make for rewarding reading for physicists who do not have the time or the mathematical preparation to delve into all the details. For such readers, the book includes interesting and accessible discussions of recent experiments on single atoms, observations on superconducting quantum interference devices, quantum information theory, quantum cryptography, quantum erasure, quantum computers, and—you guessed it—Schrödinger’s cat, just to mention a few. The author presents a fine treatment of the implications of decoherence for the various viable interpretations of quantum mechanics, and for some of the not so viable ones as well. The final chapter offers a tentative analysis of the role of the brain as the last link in the chain of devices—the von Neumann chain—that allow the human observer to form a perception in a laboratory experiment. This discussion of the “quantum brain” is necessarily as much philosophy as it is physics.
Now is a good time for physicists to take a fresh look at some of the fundamental issues in quantum mechanics (or “quantal mechanics,” as Léon Rosenfeld tried vainly to persuade us physicists to call it, a linguistic analogy to classical mechanics), and Schlosshauer’s Decoherence and the Quantum-to-Classical Transition is a welcome contribution. The index is well organized to help readers find their way around. And the more than 500 up-to-date references are a bonus for the more interested, persistent reader.
